The Dawn probe may be the feeblest spacecraft NASA's ever built, but it may usher in a new age of ultraefficient space travel

Cruise control: Artist's depiction of the craft on its way to the asteroid belt

[This article consists of a complex diagram. Please see hardcopy of magazine.] The designers at NASA are preparing to fly what may be the feeblest spacecraft they've ever built--and they couldn't be prouder of it. Never mind the decades of unmanned probes that have gone roaring into the void at tens of thousands of miles per hour, fire streaming from their tails. The new ship will putt-putt into interplanetary space under the power--if that's even the word--of an engine that accelerates by barely 15 m.p.h. (24 km/h) per day, or zero to 60 in more than half a week. Yet the...